The present invention relates generally to communications systems, and more specifically to an ISDN (integrated services digital network) multimedia communications system.
According to the current ISDN multimedia communications system, each multimedia terminal station of the system comprises a central processing unit, a video codec, a voice codec, and an ISDN interface. In addition, two LAPB (link access procedure balanced) controllers are used, one for transmitting a mode switching identifier during a transition between different modes and the other for exchanging computer-generated data during data communication mode. Frame synchronization circuitry is included to encapsulate all signals between frame sync codes. During a data communication mode, the central processing unit is linked through one of the LAPB controllers and through the frame synchronization circuitry to the interface and during a video communication mode the video codec is linked through the frame synchronization circuitry to the interface. When switching the mode of operation from video to data, the second LAPB controller is briefly switched into connection with the frame synchronization circuitry by disconnecting the video codec from it, and a data mode identifier is generated by the central processing unit and applied through the second LAPB controller and encapsulated into a specified time slot of a video frame.
Multimedia communications systems are often used as a teleconferencing system. In such applications, there is a need to access a database during video communication mode. However, the frame format of the transmitted signal prevents the terminal from accessing the database. In addition, the use of two LAPB controllers adds an extra cost to the terminal station.